This invention relates to color scanners. More specifically, this invention relates to color scanners having beam splitters that separate multi-wavelength light into component parts.
Optical scanning is a well-known and frequently used technique for converting an image on a substrate into a digital representation comprised of digital values that represent small image cells or xe2x80x9cpixelsxe2x80x9d of the image. A digital representation is useful because it can be conveniently modified, stored, copied, and/or transmitted.
Optical scanning is usually performed by placing an image bearing substrate on a stationary transparent platen, moving a light-emitting scan bar across the platen, and projecting the light reflected from the substrate and its image into a sensor assembly. The sensor assembly beneficially includes an optical sensor array that digitizes the reflected light into a xe2x80x9cscan linexe2x80x9d comprised of a plurality of pixels. As the scan bar scans the substrate the optical scanner assembly continuously produces digital representations of scan lines. When the scanning is complete the digitized scan lines represent the substrate""s image.
While the foregoing broadly describes scanning, scanning a color image is somewhat more complicated because each scan line requires a set of digital representations, one set for each color component. Typically, three color components, red, green and blue, are used. Various techniques are available for obtaining digital representations of the component color images. One approach is to make multiple scanning passes across the substrate using a different filter for each pass. This enables the production of multiple color digital representations using one optical sensor. However, since multiple scans are required this process is relatively slow. Another approach is to use multiple light-emitting scan bars and optical scanner assemblies. While fast, this approach is relatively expensive.
Another color scanning approach is to use one light-emitting scan bar, a wavelength-sensitive beam splitter, and multiple optical arrays. For example, polychromatic light (white light) can illuminate a narrow line of the substrate, the reflected polychromatic light can then be split into red, green and blue beams that are simultaneously projected onto separate light sensor arrays, and the resulting digital representations can then be obtained. This approach has the advantages of high speed at moderate cost.
One wavelength-sensitive beam splitter is a prism. As is well known, white light input to a prism is separated into its color components. Prisms operate because of the wavelength dependency of the refractive index of the prism material (usually glass). Thus by using a plurality of prisms it is possible to separate the reflected light into its various color components. However, a problem with using prisms is that the various light colors have different focal points. Since multiple optical sensor arrays are beneficially produced on a single flat substrate the use of prisms results in defocused color light. Therefore a new beam splitter that separates a multi-wavelength beam light into color components such that the color components have substantially the same optical path lengths would be beneficial.
The principles of the present invention provide for prism systems that separate a multi-wavelength light beam into a plurality of color components that have substantially the same optical path lengths. An optical system according to the principles of the present invention includes a first prism that receives multi-wavelength light, internally reflects a first component, and passes second and third component parts into a first gap. The second and third components are then reflected by a first reflector back into the first prism. The three component parts then pass into a second prism that internally reflects the first and second component parts buts passes the third component into a second gap. A second reflector then reflects the third component part back into the second prism. A third prism then receives the color components, internally reflects the third component part, and passes the first and second component parts into a third gap. A third reflector then reflects the first and second component parts back into the third prism. A fourth prism then receives the color components, internally reflects the second and third components, and passes the first component into a fourth gap. A fourth reflector then reflects the first component part back into the fourth prism. The color components then exit from the fourth prism. If the gaps are dimensioned properly the color components will have substantially the same optical path lengths.